1. Field of the Invention
This invention relates to an image scanning system for two-dimensionally scanning a light beam along a recording sheet material, and more particularly to an improvement in a sub-scanning mechanism of an image scanning system in which the main scanning is conducted using a light deflector and the sub-scanning is accomplished by mechanically moving a recording sheet material.
2. Description of the Prior Art
For recording an image or reading out a recorded image as for example in facsimiles, there has been in wide use an image scanning system in which a light beam is one-dimensionally deflected by a deflector to form a scanning line on a recording sheet material (main scanning) and the recording sheet material is moved in a direction perpendicular to the scanning line (sub-scanning), whereby a plurality of scanning lines arranged in the direction of the sub-scanning in parallel to each other are formed. In the image scanning system, the main scanning must be conducted with a constant period and the recording sheet material must be moved in the sub-scanning direction at a constant speed. When the speed at which the recording sheet material is moved in the sub-scanning direction (sub-scanning speed) varies, the density of the scanning lines fluctuates. The fluctuation in the density of the scanning lines results in fluctuation in the density of the image, thereby lowering the quality of the image. This problem is especially significant when processing images having continuous gradation. See, for example, "Various Problems in the Sub Scanning in a Cylindrical Facsimile" ("National Technical Report" Pages 550 to 558, No. 55, vol. 22, October, 1976), or "Visibility and Correction of Periodic Interference Structures in Line-by-Line Recorded Images" ("Journal of Applied Photographic Engineering" Pages 86 to 92, No. 2, vol. 2, April, 1976). Further, our experiments have revealed that when the sub-scanning speed fluctuates by 0.1% in recording an image with a density of 10 to 20 lines/mm, the obtained image exhibits visually detectable density fluctuation.
Conventionally, the sub scanning mechanism for feeding the recording sheet material at a constant speed with a high accuracy comprises a screw rod which is engaged with a nut fixed to a table for carrying a recording sheet material. By rotating the screw rod at a constant speed, the table is fed at a constant speed in the axial direction of the screw rod. This sub-scanning mechanism is disadvantageous in increasing the processing rate since the table must be returned to the original position after the scanning of one sheet material is finished and the image scanning operation must be interrupted while the table is returned. Further, the sub-scanning mechanism is expensive since the screw rod, the nut and a guide rail for guiding the table must be produced with a very high accuracy and since various complicated mechanisms are required for fixing the recording sheet material on the table, discharging the same from the table and locating the same in a predetermined position on the table. Since such complicated mechanisms are combined, the recording sheet material is apt to be positioned in the wrong position and the sub-scanning mechanism frequently gets jammed with the recording sheet material.
Thus, we have proposed, in our U.S. patent application Ser. No. 395,320 filed on July 6, 1982 (European Publication No. 0069384), an improved image scanning system which is inexpensive and able to effect sub-scanning with a high accuracy without jamming of the recording sheet material, and in which the time required for changing the recording sheet material is reduced to the minimum.
The image scanning system comprises a sub-scanning drum adapted to be continuously rotated to feed the recording sheet material in the sub-scanning direction which is perpendicular to the rotational axis of the sub-scanning drum, a main scanning system for scanning a light beam across the recording sheet material in the direction of the rotational axis of the sub-scanning drum, at least one nip roll movable between a first position in which it is pressed against the sub-scanning drum at a part near the scanning position of the light beam and a second position in which it is removed therefrom, a sheet feeding means for feeding the recording sheet material between the sub-scanning drum and the nip roll with the recording sheet material being unstrained, a stopper which is disposed downstream of the sub-scanning drum and the nip roll with respect to the feeding path of the recording sheet material and is movable into and away from the feeding path, a sheet discharging means for discharging the recording sheet material, and an associating means for associating the nip roll and the stopper with each other, said sheet discharging means being arranged so as not to act on the recording sheet material to be discharged until the scanning of the recording sheet material is completed.
In the image scanning system, said associating means need not move the nip roll and the stopper simultaneously insofar as the nip roll and the stopper move in response to the movement of each other. Further, said sheet discharging means may be provided at a position spaced from the nip roll by a distance larger than the length of the recording sheet material so as not to act on the recording sheet material while the recording sheet material is pressed against the sub-scanning drum by the nip roll. Otherwise, the sheet discharging means may be positioned spaced from the nip roll by a distance not larger than the length of the recording sheet material insofar as the scanning of the recording sheet material has been completed by the time the leading end of the recording sheet material is brought into contact with the sheet discharging means.
In the image scanning system, the recording sheet material is fed toward the stopper with the nip roll removed from the rotating sub-scanning drum and the stopper positioned in the sheet feeding path. When the leading end of the recording sheet material abuts against the stopper and the recording sheet material is positioned in a predetermined position with respect to the sub-scanning drum, the nip roll is pressed against the sub-scanning drum sandwiching the recording sheet material therebetween, whereby the recording sheet material is moved in the sub-scanning direction. The stopper is retracted from the sheet feeding path when the nip roll is pressed against the sub-scanning drum so as not to prevent the feeding of the recording sheet material at a constant speed. The sub-scanning drum is continuously rotated and the feeding of the recording sheet material is started when the nip roll is pressed against the drum sandwiching the recording sheet material therebetween. Therefore, fluctuation in the sheet feeding speed cannot occur due to overshooting or ringing at the starting time as frequently occurs in conventional systems in which the sub-scanning mechanism is actuated in response to introduction of the recording sheet material into the sub-scanning mechanism. Further, in the image scanning system, the recording sheet material is free from the sheet feeding means when it is supported on the stopper, and is free from the sheet discharging means while the scanning is effected, which also contributes to prevention of fluctuation in the sheet feeding speed. Further, since the sub-scanning drum is continuously rotated, as soon as one recording sheet material is removed from the sub-scanning drum, the next recording sheet material can be fed to the sub-scanning drum. Thus, the time required for changing the recording sheet material is reduced to the minimum.
Although being basically satisfactory, the image scanning system involves some problems in practical use. For example, there is a possibility that the recording sheet material is scratched by the nip roll when the recording sheet material is pressed against the sub-scanning drum by the nip roll to start scanning, since the sub-scanning drum is continuously rotated at a constant speed. Further, in the image scanning system, the distance between the leading end of the recording sheet material and the first scanning line formed on the sheet material is determined depending upon the distance between the stopper and the scanning position which are both fixed. However, the size of the sheet material is not always constact and different size of recording sheet materials are used according to the kind of image to be recorded. The size of images also varies and different size of images are recorded on recording sheet materials of the same size. It is not preferable, from the viewpoint of efficiency of recording and reading out, to start scanning from a position spaced by a fixed distance from the leading end of the recording sheet material irrespective of the size of the recording sheet material or the size of the image to be recorded thereon. Practically, the distance between the first scanning line and the leading end of the recording sheet material is preferred to be as small as possible so that a larger amount of information can be carried thereon.